1. Field of the Invention
The present invention relates to novel high energy density electrochemical cells and more particularly to novel rechargeable, high energy density electrochemical cells having alkali metal anodes and containing electrolytes which include organic solvent and alkali metal salts of complex anions containing heteroatom substituents.
2. Prior Art
A recently developed rechargeable, high energy density electrochemical cell consists of an alkali metal material as the anode-active material, a transition metal chalcogenide as the cathode-active material, and a nonaqueous electrolyte. More specifically, preferred cells consist of lithium anodes, titanium disulfide cathodes and nonaqueous electrolyte compositions consisting of various lithium salts, such as LiClO.sub.4, dissolved in organic solvents, such as propylene carbonate, tetrahydrofuran, dioxolane, and mixtures of dimethoxyethane and tetrahydrofuran, and containing various stabilizing additives.
Important features of these cells include their ability to be repeatedly discharged and charged. Theoretically, cycling by discharging and charging should be possible indefinitely, but in practice indefinite cycling is not realized. Dendritic growth on the anode during charging and degradation of the cathode material are sometimes limiting factors in the amount of cycling to which a cell can be subjected. However, the electrolyte, particularly nonaqueous electrolytes, can at times be the limiting factor. The effects of a particular electrolyte composition on the electrochemical performance of a cell may be significant due to its relative stability or it may be due to other factors. One particular electrolyte composition might be highly effective with a given anode-cathode couple but be ineffective for another couple, either because it is not inert to the second couple or because it reacts with itself under the conditions present during cycling. Furthermore, even when a particular electrolyte composition is effective in a given cell, it may nonetheless be undesirable for other reasons. For example, the sometimes preferred LiClO.sub.4 based electrolyte creates a potential explosion hazard.
It is believed that the advantageous electrolytes used in the cells of the present invention have not been heretofore made or discovered and that no electrolytes existed heretofore which would render the cells of the present invention containing these electrolytes obvious. U.S. Pat. No. 3,904,692 (Lassau et al) describes various compounds as catalysts for dehydrogenation of secondary alcohols and among those included in the generic expression disclosed therein are LiAlH(CH.sub.3).sub.2 [N(CH.sub.3).sub.2 ] and LiAlH(C.sub.2 H.sub.5).sub.2 [N(CH.sub.3).sub.2 ]. However, these compounds contain hydrogen as one of the metal substituents and are therefore not similar to the salts used in the electrolytes of the cells of the present invention which contain no hydrogen substituents. Also, Smith et al, J. Am. Chem. Soc., Vol. 73, June 1951, p 2751 et seq., describe compounds such as NaB(CH.sub.3).sub.3 [NH.sub.2 ] and LiB(CH.sub.3).sub.3 [N(H)C.sub.2 H.sub.5 ]. However, these compounds of Smith et al have hydrogen substituents on the nitrogen atom, i.e., the compounds contain primary amide (primary ammonobase) substituents, whereas the salts of the electrolytes used in the cells of the present invention contain no hydrogen substituents on the nitrogen atom, i.e. the present invention cell electrolyte salts contain secondary amine (secondary ammonobase) substituents. Additionally, the Smith et al compound of the formula LiB(CH.sub.3).sub.3 [N(H)C.sub.2 H.sub.5 ], believed to be closest to the salts used in the present invention, was found to be unstable and decomposed on standing at -33.degree. C. U.S. Pat. No. 3,734,963, now Reissue No. 28,456 (Langer et al) describes various alkali metal organometallic compounds which are chelated with certain bifunctional complexing agents. However, neither the organometallic compounds per se nor the chelate complexes of the Langer et al patent contain those heteroatom substituents of the salts used in the present invention. Recent studies have been made directed to LiB(C.sub.6 H.sub.5).sub.4 electrolyte systems by Szwarc et al, J. Phy. Chem., Vol. 69, p 608 et seq. (1965), but these systems have been found to have low solubility and high resistivity.
U.S. Pat. No. 3,764,385 (Langer et al) describes alkali metal organometallic compounds which are complexed with certain complexing agents having bifunctionality, and the resulting chelate complexes are described as useful electrolytes for primary and secondary batteries. However, the patent does not teach compounds such as those used in the present invention having the claimed heteroatom substituents. In view of the differences between the novel salts used in the present invention and the compounds and complexes of Langer et al, it is believed that the Langer et al patent does not teach or render obvious the electrolyte compositions of the present invention. U.S. Pat. No. 4,060,674 formerly copending U.S. application Ser. No. 750,517, filed on Dec. 14, 1976, describes the use of various alkali metal organometallic compounds, e.g. LiB(CH.sub.3).sub.4, as useful electrolyte components in organic solvents, but again such compounds are different from those used in the present invention in that they do not contain the presently claimed heteroatom substituents.
For these reasons, novel but effective lithium salt-containing electrolyte composition-containing alkali metal anode type cells are desirable.